Lithium-ion rechargeable battery cells currently use a carbon/graphite-based anode. The basic composition of a conventional lithium-ion rechargeable battery cell including a graphite-based anode electrode is shown in FIG. 1. A battery may include a single cell but may also include more than one cell.
The battery cell generally comprises a copper current collector 10 for the anode and an aluminium current collector 12 for the cathode, which are externally connectable to a load or to a recharging source as appropriate. It should be noted that the terms “anode” and “cathode” are used in the present specification as those terms are understood in the context of batteries placed across a load, i.e. the term “anode” denotes the negative pole and the term “cathode” the positive pole of the battery. A graphite-based composite anode layer 14 overlays the current collector 10 and a lithium containing metal oxide-based composite cathode layer 16 overlays the current collector 12. A porous plastic spacer or separator 20 is provided between the graphite-based composite anode layer 14 and the lithium containing metal oxide-based composite cathode layer 16: a liquid electrolyte material is dispersed within the porous plastic spacer or separator 20, the composite anode layer 14 and the composite cathode layer 16. In some cases, the porous plastic spacer or separator 20 may be replaced by a polymer electrolyte material and in such cases the polymer electrolyte material is present within both the composite anode layer 14 and the composite cathode layer 16.
When the battery cell is fully charged, lithium has been transported from the lithium containing metal oxide via the electrolyte into the graphite-based anode where it is intercalated by reacting with the graphite to create a lithium carbon compound, typically LiC6. The graphite, being the electrochemically active material in the composite anode layer, has a maximum theoretical capacity of 372 mA h g−1. For the avoidance of doubt, the term “active material” is taken to describe any material into which lithium ions can be inserted and extracted during operation of the battery cell.
It is well known that silicon can be used instead of graphite as the active anode material (see, for example, Insertion Electrode Materials for Rechargeable Lithium Batteries, M. Winter, J. O. Besenhard, M. E. Spahr, and P. Novak in Adv. Mater. 1998, 10, No. 10 and Nano- and bulk-silicon-based insertion anodes for lithium-ion secondary cells, U. Kasavajjula, C. Wang and A. J. Appleby in J. Power Sources 163, pp 1003-1039, 2007). It is generally believed that silicon, when used as an active anode material in a lithium-ion rechargeable cell, can provide a significantly higher capacity than the currently used graphite. Crystalline silicon, when converted to the compound Li21Si5 by reaction with lithium in an electrochemical cell, has a maximum theoretical capacity of 4,200 mAh/g, considerably higher than the maximum capacity for graphite. However there are several different Li—Si alloys that can be formed by lithium insertion, depending on e.g. temperature, crystalline state, charge voltage and charge rates. For example, at room temperature it is believed that the maximum achievable capacity is close to 3,600 mAh/g with the alloy Li15Si4. (“Structural changes in silicon anodes during lithium insertion/extraction”, M. N. Obrovac and Leif Christensen, Electrochem. & Solid State Lett., 7, A93-A96, 2004). Thus, if graphite can be replaced by silicon in a lithium rechargeable battery, a substantial increase in stored energy per unit mass and per unit volume can be achieved. Unfortunately silicon anode material in Li-ion cells undergoes a huge volume change between the charged and the discharged states associated with the insertion and removal of lithium ions into the silicon material during the charging and discharging stages of the cells. The volume of a fully lithiated Li—Si alloy can be 3-4 times larger than the unalloyed silicon volume. This is much larger than the volume change seen in carbon anodes. As a consequence of such expansion and contraction, which on each cycle causes mechanical degradation of the silicon material and electrical isolation of sections, the electrodes can have a short cycle life.
It is believed that the likelihood of structural breakdown of the anode active material during expansion and contraction can increase if crystalline and amorphous Li—Si alloy phases are allowed to co-exist during a charge-discharge cycle. If the initial anode material is crystalline silicon then, during the first charge cycle as lithium is inserted, it loses its crystalline structure and becomes an amorphous Li—Si alloy. If at this amorphous stage, the anode is then delithiated, i.e. the cell is discharged, then the silicon anode material remains amorphous. However, if the silicon anode material proceeds to full lithiation, then as the anode potential approaches zero volts, a crystalline Li15Si4 phase forms. On discharge (delithiation), this crystalline alloy phase converts back into the amorphous Li—Si alloy. Although this crystalline phase provides the highest charge capacity, it is preferable to avoid its formation because of the additional stresses induced in the anode material from the repeated crystalline to amorphous transitions in subsequent cycles. Formation of the crystalline phase can be prevented by avoiding excessive charging of the silicon anode material and setting a lower voltage limit on the anode during charging (that is, not allowing it to be charged beyond a lower voltage level which depends, amongst other things, on the internal cell resistance but is typically in the range of 15-50 mV). Setting a limit on the charge level of the silicon anode material also helps to control mechanical stresses in the anode and minimise cracking of the silicon material. For this reason it is preferable not to charge the silicon material above 3,400 mAhr per gram of silicon, and most preferable to set an upper limit of no more than 2,500 mAhr/g. This equates to a charge that is less than 80%, and preferably no more than 60%, of the theoretical maximum of an active mass consisting wholly of silicon, and such percentages also apply in the case of an active mass formed of a mixture of silicon and one or more other active materials, e.g. carbon; the theoretical maximum charge for carbon is 372 mAh/g and for silicon is 4200 mAh/g.
Another factor affecting cell performance is the formation a solid electrolyte interface (SEI) layer on the silicon surface. Initially the surface of the silicon material has a thin native oxide layer on it which has a low conductivity. During the first charge, this layer is replaced by an SEI layer of higher ionic conductivity formed from reactions with the electrolyte and reduction of the solvents. The SEI can be composed of various different products, for example Li2CO3, LiF, Li2O, lithium alkyl carbonates, polymeric hydrocarbons and others. Each product will start forming at different stages of the charging process, dependent on the anode potential. Some illustrative reactions are as follows:—RO.CO.OR′+2Li++2e−→ROLi(s)+R′OLi(s)+CO(g)RO.CO.OR′+2Li++2e−→Li2CO3(s)+R.R′(g)LiPF6LiF(s)+PF5 Li2CO3(s)+PF5→2LiF(s)+POF3+CO2(g)POF3+nLi++ne−→LiF(s)+LixPOFz(s)where R and R′ are generally alkyl groups; RO.CO.OR′ is present in the cell as part of the electrolyte to provide a solvent for a lithium salt, e.g. LiPF6. A stable SEI layer with good ionic conductivity that can withstand the volume changes is essential to the proper working of the cells, and in this regard certain SEI products are much better than others. For example, one preferred component of the SEI is generally perceived to be Li2CO3.
A drawback of the SEI formation is that it consumes some of the lithium and electrolyte components, and ties them up in the system, preventing them from contributing to the charge capacity of the cell. Excessive SEI formation will lead to an increased ionic resistance and degrade the cell performance. Therefore it is preferable to control the surface area and the surface area to volume ratio of the silicon anode material. After the first cycle, the repeated expansion and contraction of the silicon during cycling can cause cracks in the SEI, exposing fresh silicon surfaces, leading to production of more SEI layer, consuming further liquid electrolyte and lithium. This reduces the cell charge/discharge efficiency, can cause the cell to dry out, and further reduces available cycle life. The cracking typically occurs at a low anode potential during charging (when the volume expansion is largest) and at this point numerous SEI products are able to form at once on the exposed surface. It is thought that this can lead to the formation of poor quality SEI layers and to avoid this it is desirable to have preferred SEI products form over the others.
It should be recognised that the main part of the SEI layer is formed during the first few charge cycles and this process contributes to the irreversible capacity and lower charge efficiencies typically experienced in the early cycles. In subsequent cycles, new areas of SEI will be continually formed where silicon is exposed through cracking or where the original SEI layer cracks or degrades and this process contributes to running losses of lithium and helps determine the running charge efficiency. The quality of the original SEI layer will have a significant influence on the quality of the new SEI formation during later cycles. An SEI layer preferably has the following properties:                Uniform (smooth), non-porous covering of every part of the exposed silicon surface (preferably a full covering formed during the first cycle)        High ionic conductivity        Low electronic conductivity        Relatively thin        Stable        Flexible—can stretch with the silicon material as it expands and contracts (the more it cracks the more SEI will be formed and the more lithium is consumed)        
The SEI formation process can be influenced through the use of electrolyte additives. The use of cyclic carbonates containing a vinyl group such vinylene carbonate (VC), halogenated cyclic carbonates such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC), CO2, silyl esters such as sultones and esters of phosphoric and boric acid have been used as electrolyte additives.
Any additive to the electrolyte must not adversely affect the properties of the electrolyte; an electrolyte can be adversely affected by the addition of additives and, even with the addition of additives, the electrolyte should:                not be overly depleted of lithium by the additions        maintain a high ionic conductivity        not be too viscous        operate safely at temperatures experienced in the cells        electrochemically compatible with the cathode material (an SEI layer forms on the cathode too and we do not want the cathode performance to be reduced)        
The concentrations of additives should be such that, on the one hand, they are effective while, on the other hand, they should not compromise the role of the electrolyte and especially the above properties.
Vinylene Carbonate (VC):
is known as an electrolyte additive for graphite anode cells to improve the charging and discharging performance of the cells. See, for example, U.S. Pat. No. 7,862,933, JP 04607488 and J. Electrochem. Soc., 156(2), A103-A113 (2009). We believe that it (VC) changes the composition or properties of the SEI layer on the graphite anode, which has a completely different composition to the SEI on a silicon-based anode. Typically, the VC content of the electrolyte in cells having a graphite anode is around 2 wt %.
Fluoroethylene Carbonate (FEC):
is also known as an additive to battery electrolytes, as described below and in, for example, U.S. Pat. No. 7,862,933, US 2010/0136437, JP 04607488, JP 2008234988 and US 2007/0072074.
Electrolyte solutions containing silyl, borate and phosphate esters as additives are disclosed in JP 04607488, JP 2008234988 and US 2010/0136437.
Further, the structure of the anode material greatly affects the formation of the SEI layer. The present invention is concerned with a structured silicon material that is open (i.e. contains space within the mass) and allows the growth of the SEI layer and the expansion of the silicon-containing anode during charging, i.e. lithiation, of the anode. The porosity of an anode made of such structured silicon may be relatively high, e.g. >30% volume porosity or >40%. The open structure can be brought about by the structure of the particle itself, e.g. it could have structural elements, e.g. pillars or similar protrusions, on its surface that provide spaces between the elements that allow for the growth of the SEI and the expansion of the silicon during lithiation. In another embodiment, the structure of the particle could contain voids within it that fulfil the same function. Alternatively, the particles could be shaped such that they allow space between the particles for the growth of the SEI and the expansion of the silicon during lithiation when deposited onto the current collector of an anode. Such particles will generally be elongated and have an aspect ratio (ratio of the smallest to largest dimension) of at least 5 and optionally at least 10, e.g. at least 25. Such structures have a more convoluted surface morphology than simple (i.e. unstructured) silicon particles or films and have lots of sharp corners and changes in surface directions, which makes it more difficult to form a thin, flexible (elastic), non-porous SEI coating over the whole exposed silicon surface. The porous nature of a mass formed from the structured silicon increases the likelihood of having void spaces created within the mass that have narrow access paths for the electrolyte and so it is particularly important that the viscosity of any electrolyte used should not be so high that the electrolyte cannot migrate into and out of such void spaces, which would render such spaces dead.
Chan et al in Journal of Power Sources 189 (2009) 1132-1140 investigated the formation of an SEI layer on silicon nanowires during lithiation in a standard electrolyte free of additives. They found that the morphology of the SEI layer was very different compared to that typically found on thin film anodes: less uniform, a reduced level of LiF and some particle deposits adjacent to the nanowires on the anode substrate rather than adhered to the surface of the silicon. This shows that the morphology and composition of the SEI layer for highly structured, porous silicon material is very different to other silicon anodes where additives have previously been used.
The structured silicon material used in the electrodes of the present invention therefore provides a special problem to find additive(s) and a range of additive concentrations to achieve the properties outlined above. It will be appreciated from the foregoing that there is a need for electrolyte solutions, which facilitate the formation of a stable SEI layer on the surface of structured electroactive materials, particularly structured silicon materials.
The relevant prior art of which we are aware is:
U.S. Pat. No. 7,476,469 discloses a rechargeable (secondary) battery that includes an anode, a lithium cobalt oxide cathode and an intervening body of non-aqueous electrolyte. The anode may be, amongst other materials, an amorphous silicon thin film layer that is sputtered onto a current collector and is typically 1-20 μm thick; although the specification also teaches the use of microcrystalline silicon, no examples are given of the use of this. The electrolyte contains cyclic or chain carbonates, including ethylene carbonate. The electrolyte can also contain vinylene carbonate, which is said to improve the charge-discharge cycle performance characteristics. The amount of vinylene carbonate is stated to be 0.5-80% by volume of the other components of the electrolyte. However, the teaching of this citation is limited to the type of anode material used (a thin film of amorpohous silicon or fine crystalline silicon with nm-sized grains).
US-2009/0053589 discloses a battery cell having a cathode, an anode and electrolyte. The anode contains, as active material, a powdered or thin film special alloy, e.g. of silicon, tin and a transition metal, in various microcrystalline and amorphous phases, which are expensive. The microcrystalline materials may have a crystallite dimension of 5-90 nm. The electrolyte can include various cyclic carbonates, including VC or FEC. In the Examples, the amount of VC or FEC is 10% of the electrolyte, which is said to reduce the capacity loss in the first cycle. Again this teaching is limited to the special powdered or thin film alloy anode material used.
US2009/0305129 discloses a lithium secondary battery having an anode that includes polycrystalline particles of silicon or of a silicon alloy prepared by thermal decomposition. In order to increase the cycle performance of the battery, the silicon anode material must have a crystallite size in the nanometer range (less than 100 nm) within silicon particles having a diameter of 3-30 μm. The specification teaches that CO2 (in an amount of about 0.4 wt %) and/or a fluorine-containing carbonate, e.g. FEC (in an amount of about 10 wt %), may be added to an ethylene carbonate/diethylene carbonate electrolyte since they are said to allow the reaction of the silicon particles with lithium to happen smoothly and increase the number of charging/discharging cycles before failure. The purity of the silicon is 95% or higher.
Nam-Soon Choi et al in Journal of Power Sources 161 (2006) 1254-1259 discloses that the lifetime of a thin film silicon electrode having a thickness of 200 nm can be improved by the addition of 3% FEC to an ethylene carbonate/diethylene carbonate electrolyte.
L. El Ouatani et al in Journal of the Electrochem. Soc. 156 (2009) A103-A113 discloses that the addition of VC forms a thinner SEI layer on graphite anodes than layers formed with no VC, and that VC introduces extra oxygen containing compounds into the SEI. One of these compounds is a polymer derived from VC. This is clearly different from the lithium containing dimer which is one of the main reaction products from the reduction of ethylene carbonate (EC) solvent within the electrolyte, which occurs according to equation 1:2EC+2Li++2e−→LiO.CO.O.CH2.CH2.O.CO.OLi(s)+C2H4(g)We believe that the VC is reduced according to the equation 2:
to form the polymer identified.
Both of the above reactions are believed to proceed via a radical reduction mechanism. It is possible that some of the lithium conduction mechanism of the SEI layer comes through oxygen containing hydrocarbons, in a manner analogous to the conduction mechanism in poly (ethylene oxide): LiX materials. In this case, the lithium conduction is likely to be better with the polymer derived from VC, compared to the dimer derived from EC.
Libao Chen et al. in Journal of Power Sources 174 (2007) 538-543 discloses that the presence of 1 wt % VC to the electrolyte improved the cycling performance of a thin silicon film of 250 nm thickness compared to similar film with an electrolyte free of VC. The improvement was attributed to a thinner, more uniform SEI layer on the surface of the silicon anode with VC additive. The SEI layers were found to contain significant amounts of LiF. Unexpectedly, small quantities of SiOx were also observed and this is thought to be due to interaction of the electrolyte with lithiated silicon via holes in the SEI layer. The silicon material must have a small particle size because the width of the whole film is only 250 nm.
Sheng Shui Zhang in Journal of Power Sources 162 (2006) 1379-1394 discloses that FEC can decompose to VC and HF. The presence of hydrogen fluoride would normally be considered detrimental to lithium ion battery performance, because it reacts with lithium carbonate to produce lithium fluoride, water and carbon dioxide. However, there are reports that HF improves the quality of SEI layers on lithium metal electrodes, smoothing the deposits and reducing the risk of dendrite formation.
It has been suggested that the internal stresses generated in the silicon material from repeated expansion and contraction can be reduced by using silicon material of very small (sub-micron) dimensions. However this also causes disadvantages: in the case of thin films (see for example the Libao Chen et al. article discussed above), it reduces the available anode capacity per unit volume; in the case of sub-micron particles it increases the silicon surface area to volume ratio, which increases the proportion of SEI formed during operation and the consumption of lithium and the electrolyte. The long range connectivity within the active material is also poorer with spherical nano-particles.
Batteries suitable for use in hybrid electric vehicles are disclosed in U.S. Pat. No. 7,862,933. Each battery comprises a graphite based anode, a cathode, a separator and an electrolyte solution. The electrolyte solution comprises as a base solvent a mixture of a cyclic carbonate, a linear or chain carbonate and a cyclic carbonate containing a vinyl group; although these carbonate species may include halogen substituents, such substituents are not exemplified and batteries containing from 0.2 to 0.4 vol % VC only as substituent are exemplified.
US 2010/0124707 discloses batteries suitable for use in portable electronic devices. Each battery comprises a cathode, an anode and an electrolyte solution. The anode comprises a current collector having a layer of an anode active material applied thereto. The anode active material is disclosed as containing a plurality of spherical and non-spherical anode active material particles having silicon as an element and is prepared by spraying a silicon-containing material onto the surface of a surface roughened current collector. An electrolyte solution typically contains a mixture of a cyclic carbonate and a linear or chain carbonate, both of which may contain a halogen as a substituent or a halogenated substituent. Cyclic carbonates including a vinyl group, sultones and acid anhydrides may be used as additives. Examples of simple and complex lithium salts as the electrolyte salt are disclosed. There are no examples of batteries containing a mixture of a cyclic carbonate containing a vinyl group or a halogenated cyclic carbonate. Further there are no examples of base solvents other than unsubstituted cyclic carbonates and linear carbonates.
Rechargeable lithium ion batteries including an anode made by sintering a layer comprising particles of an active material including silicon and/or silicon alloy and a binder on a surface of a current collector are disclosed in US 2006/0003226. The anode active particles typically have an average diameter of not more than 100 μm, suitably not more than 50 μm and preferably not more than 10 μm. The batteries also include a non-aqueous electrolyte comprising a mixture of a cyclic and a linear carbonate and having carbon dioxide (CO2) dissolved therein. The dissolved CO2 is believed to limit expansion of the anode material through the formation of a stable SEI layer on the electrode surface. The electrolyte may also optionally contain at least 1 wt % of fluorinated cyclic or chain carbonate; no examples of electrolyte solutions including a fluorinated solvent are included. Batteries including electrolyte solutions containing CO2 were observed to exhibit longer cycle lifetimes compared to batteries in which the electrolyte includes VC as an additive.
U.S. Pat. No. 7,674,552 discloses a lithium ion battery having a lithium fluoride-lithium hydroxide coated anode, a cathode and an electrolyte. The anode is formed by depositing a layer of an anode active material onto a current collector using techniques such as vapour deposition, electroplating or by deposition of a slurry comprising a dispersion of a particulate electroactive material; only vapour deposition techniques are disclosed. The electrolyte suitably comprises a solution of LiClO4 in a solvent comprising a mixture (typically a 1:1 mixture) of a fluorinated cyclic carbonate and a linear or chain carbonate; other suitable solvents include sulpholane, acetonitrile and VC. The anode coating is formed by charging the battery including this electrolyte over at least 30 cycles. The ratio of Li2F+ to Li2OH+ in the coating was at least 1. Batteries having a higher Li2F+ to Li2OH+ ratio exhibited superior charge and discharge efficiencies over 30 cycles.
US 2010/0136437 discloses a method of forming a fluoride coating on a copper coated particulate silicon electroactive anode material by charging a battery including the anode in an electrolyte solvent including a cyclic fluoride containing carbonate over more than 100 charge/discharge cycles; the first charge discharge operation is carried out at a charge rate of between 0.005 and 0.03 C. The electrolyte suitably contains 15 to 40 vol % of a fluorinated cyclic carbonate such as fluoroethylene carbonate and may optionally further contain 0.5 to 5 wt % VC, 0.1 to 1.5 wt % 1,4-butanediol dimethylsulfonate and/or 0.1 to 1 wt % of dimethylsulfone. There are, however, no examples of electrolyte solvents including one or more of VC, 1,4-butanediol dimethylsulfonate and/or 0.1 to 1 wt % of dimethylsulfone. Examples of batteries prepared in accordance with US 2010/0136437 include silicon particles having an average particle size of between 0.3 and 3 μm and an electrolyte solution comprising a mixture of diethylene carbonate (DEC) with either ethylene carbonate (EC) or fluoroethylene carbonate (FEC).
JP04607488 discloses lithium ion batteries including an anode having an anode active material that can be formed from a material such as silicon, tin or graphite or oxides thereof; only anodes comprising an electroactive graphite are disclosed. The battery further includes a cathode, a separator and an electrolyte. The electrolyte suitably comprises a base solvent comprising a mixture of a cyclic carbonate and a chain carbonate in a ratio in the range 5:95 to 80:20, preferably 4:6 and a 0.1 to 10 wt % of silyl ester additive. Silyl esters of carbonic, phosphoric and boric acid are envisaged. Further optional additives include 0.2 to 0.5 wt % of a tetrafluoro-borate and 0.1 to 10 wt % of a cyclic carbonate including a vinyl group. The silyl ester is believed to reduce the irreversible first cycle losses of the battery. The borate contributes to maintaining the viscosity of the electrolyte solution and the optional presence of VC is believed to reduce the extent of reductive cleavage of the electrolyte solvents.
JP 2008234988 discloses an anode having an active material applied to a copper current collector. The active material includes a silicon base layer to which is applied one or more coating layers comprising an alkali metal salt of a transtition metal fluoride such as fluorides of scandium, ytterbium, titanium and hafnium. The anodes are included in a battery structure together with a cathode, a separator and an electrolyte. The electrolyte typically comprises a mixture of cyclic and chain carbonates as base solvents and 1 to 2 wt % of an additive such as a sultone, succinic acid, succinic anhydride or sulfobenzoic anhydride to improve the cell performance by between 1 and 5%; this effect is most noticeable when the electrolyte is a mixture of DEC and FEC.
Lithium ion batteries comprising a thin film silicon-based alloy material applied to a negative electrode are disclosed in U.S. Pat. No. 7,659,034. 0.05 wt % or more of carbon dioxide or vinylethylene carbonate may optionally be added to the electrolyte solvent to prolong cell life and to enhance capacity retention.
US 2007/0037063 discloses a rechargeable lithium ion battery including an electrolyte solution containing an ethylene carbonate compound. Typically the base electrolyte solvent comprises a mixture of a cyclic carbonate and a chain or linear carbonate in a 30:70 ratio. The electrolyte solvent optionally further includes 0.1 to 15 wt % FEC and optionally up 3 wt % of VC. The performance of a cell comprising an electrolyte including an FEC additive was associated with a better irreversible cycle efficiency compared to cells including electrolytes.
US 2007/0072074 discloses a method for reducing gas generation in lithium ion batteries by including 2 to 10 wt % of an FEC additive in the electrolyte solution in combination with a 0.1 to 1M of an electrolyte salt including LiBF4. The electrolyte may also contain up to 2 wt % of VC. Silicon-based anodes can be prepared by either applying a slurry of a silicon-containing particulate material to the current collector or by using vapour deposition techniques to form a silicon-containing thin film on a current collector. The preparation of anodes using vapour deposition techniques only is disclosed. There is no disclosure of the size or shape of the silicon-containing particles used to prepare the anodes.
US 2008/0241647 discloses a cylindrical lithium ion battery comprising a cathode, an electrolyte and an anode. The anode comprises an anode active material comprising silicon-containing and/silicon alloy particles having a principle diameter in the range 5 to 15 μm. The electrolyte suitably comprises a base solvent comprising a mixture of a cyclic and a chain carbonate and further comprises up to 0.4 wt % and optionally up to 10 wt % of CO2. US 2004/0151987 discloses a battery comprising a cathode, an anode, a separator and an electrolyte. The anode is suitably formed from a silicon-containing slurry or from a vapour deposited silicon thin film onto the surface of the current collector. Silicon-containing slurries suitably contain silicon particles having a diameter of around 10 μm. The electrolyte suitably comprises a base solvent comprising a mixture of a cyclic carbonate and a chain carbonate in a ratio of 3:7 and 0.1 to 30 wt %, preferably 5 wt % of a vinylene carbonate additive.
There is, as indicated above, a need for lithium ion batteries, which contain electrolytes which contribute to the formation of a strong and flexible SEI layer and which maintain the charge and discharge capacity over a prolonged charge/discharge life. The present invention addresses these needs.